Mkrriam. | 



Sigmogomphius Le Contei. 



367 



the crown to meet a similar fold from the opposite side. In the 

 second and third it crosses the triturating surface, touching or almost 

 touching the exterior wall. The presence of the fold is marked on 

 the inner side of the tooth by a strong, sharp groove reaching almost 

 to the end of the larger root. 



On the outer side one, two, or three folds may be present. 

 The premolars have three, the first molar one, and the second 

 molar in the right series one, in the left series two. Of the three 

 folds present in the premolar the middle one is the strongest, 

 reaching across the tooth to the inner wall. The anterior outer fold 

 comes in contact with the inner loop about the middle of the tritu- 

 rating surface, and shows a decided thinning of the wall at the point 

 of contact, so that the two folds have almost united to form a trans- 

 verse lamella. The third, or posterior fold, occurring in the premo- 

 lars and the second molar of the left side, is small but distinct. On 

 molars one and two the large exterior fold present corresponds to 

 the middle one of the premolar. In these two there seems to be 

 nothing corresponding to the anterior outer fold of the premolar. 

 In Fig. \,b, the first molar tooth of the right side is shown in trans- 

 verse section, one-fourth of the distance from the lower end. Here 

 the anterior outer fold is seen to be an island, while the wall between 

 it and the end of the inner fold is very thin. It seems possible that 

 the island could unite with the distal end of the inner fold, savin"- 

 it the length found in molars 1 and 2. The upper incisors, Fig. 1, a, 

 were both present and differ little, if at all, from those of Castor or 

 Eucastor. 



COMPARISON WITH OTHER CASTOROID GENERA. 



In comparing this specimen with the other genera of the Cas- 

 toridae, there is little in the bones of the skull, which have been 

 found, to show its relations, though the palatal and nasal regions 

 are rather more like Steneofiber than Castor. Characters of great 

 comparative value are, however, found in the highly developed 

 molars. The number of molar teeth, three, is evidently important, 

 since the normal number for the Castorida; is four. In Rlylagaulus, 

 Cope, there is an exception, reduction to three or two having taken 

 place. In Eucastor tortus, Leidy, described from beds of nearly the 



